No immediate evidence of 'collapse crater' at nuclear test site: 38 North

Satellite imagery showed numerous landslides throughout North Korea's nuclear test site and beyond, but there appears to be no evidence of a "collapse crater", according to 38 North, the website of a US research institute.

The area of these surface disturbances, which include numerous areas of pre-existing gravel and scree fields that have apparently been lofted in place from the tremors, is centered about Mount. Mantap, which is 2,205 meters high, 38 North said.

"These disturbances are more numerous and widespread than what we have seen from any of the five tests North Korea previously conducted." However, there does not appear to be any evidence of a collapse crater, as might have been suggested from the post-test tremor, 38 North said.

The extreme heat and pressure of an underground nuclear explosion cause changes in the surrounding rock which is vaporized, forming a cavity. The rock above the cavity may collapse, forming a rubble chimney, and if this chimney reaches the surface, a bowl-shaped subsidence crater may form. If a test is done at sufficient depth, there will be no release of radioactive materials to the atmosphere.

Meanwhile, the Nuclear Safety and Security Commission, a Seoul-based nuclear safety agency, said it has failed to detect any traces of radioactive materials including xenon gas in in tests of soil, water and air samples.